Up to now, energies are emitted, after being used in factories and power plants, in transportation vehicles like automobiles, and also in information processing devices like computers, into the environment in the form of waste heat. Thus, thermoelectric conversion devices capable of recycling and converting such waste heat again to energy attract extensive attention.
A thermoelectric conversion device of the type that uses Seebeck effect is capable of recovering and converting waste heat of various systems to electric power while not needing working fluids or complex driving mechanisms, in contrast to other conventional thermoelectric conversion systems.
Particularly, strontium titanate (hereinafter “STO”), which has been much studied conventionally in relation to ferroelectric devices, is free from rare or toxic materials such as tellurium, bismuth, and the like used in conventional thermoelectric conversion devices and at the same time is capable of providing a large Seebeck coefficient S that reaches as much as 0.8 mVK−1 at 300K and further a power factor, defined as S2σ (PF=S2σ), of 30-40 μW/cm·K2. Thus use of STO as the material for a thermoelectric conversion device draws much attention. Here σ stands for electrical conductivity of the thermoelectric conversion device. The power factor PF can also be represented also as S2qnμ (PF=S2σ=S2 qnμ), wherein n stands for carrier concentration per unit volume, q stands for carrier electric charge and μ stands for carrier mobility.
However, in the system of STO, while the power factor PF can reach the value of 35 μW/cm2K-40 μW/cm2K, a thermal conductivity κ takes a large value of 11 WmK when the STO is used in the form of a bulk crystal, and the value of performance index ZT defined by
                    ZT        =                                            S              2                        ⁢            T                                ρ            ⁢                                                  ⁢            κ                                              Eq        .                                  ⁢                  (          1          )                    is rather limited. In Eq. (1), it should be noted that T is the absolute temperature, S is the Seebeck coefficient of STO, ρ is the resistivity of STO and κ is the thermal conductivity of STO.
Thus, in order to attain a large performance index ZT for such a thermo-electric conversion device of STO, it is necessary to form the STO to have a high electric conductivity and at the same time low thermal conductivity κ.